Silicon wafers useful in the manufacture of semiconductor devices require close scrutiny to detect defects as soon as possible in the manufacturing process. Several apparatus are known in the art for detecting microscopic defects on the surface or near the surface of such devices. One such apparatus as described as U.S. Pat. No. 4,314,763 issued on Feb. 9, 1982 to E. F. Steigmeier, et al. utilizes a laser beam that is scanned over the surface of a wafer and includes means for detecting scattered radiation from the wafer surface. The specular reflection is blocked from the detection device by suitable arrangement of the lenses and spatial filters. If the surface of the wafer has an imperfection such as dirt, hills, scratches and the like, the laser beam will be scattered from the imperfection. There are also scattering processes such as Raman scattering, etc., which occur, but the intensity of the light due to such scattering effects is usually negligible. The scattered light from the wafer is collected from about the main axis of the lens and is focused on a detector. The scattered light is converted to electrical impulses which can be counted or, in the alternative, can be displayed as a bright spot on an oscilloscope or other monitor.
In our copending U.S. patent application Ser. No. 244,060, filed Mar. 13, 1981, entitled "METHOD FOR DETERMINING THE QUALITY OF LIGHT SCATTERING MATERIAL", now U.S. Pat. No. 4,391,524 issued July 5, 1983 we describe the use of apparatus described in our aforementioned patent to determine the relative quality of wafers compared to each other or to a reference standard. The crystalline or structural quality of the material is determined by adjusting the threshold of intensity of detected scattered light so that the display of the detected signal on a visual display is sufficient to provide a full or "blossom" display of the device. The value of the threshold adjustment is a direct measurement of the crystalline or structural quality of the material. When the surface of the material being examined includes a pattern such as that found on patterned wafers, or indeed on any surface that has a diffraction grating pattern, such as a grooved video disc, the diffracted beams mix with the scattered signals to make it difficult, if not impossible, to determine defects and dust on the surface or indeed to determine the crystalline or structural quality of the material.